Vehicles that are powered by natural gas are typically considered to be more environmentally friendly than petrol or diesel fuelled vehicles. The carbon emissions of natural gas vehicles are considerably lower than that of vehicles powered using liquid fuels such as petrol or diesel. Traditionally there is also a cost saving in relation to the price of natural gas when compared to diesel fuels.
Spark ignition (petrol) engines may be modified relatively simply to operate on natural gas or liquid petroleum gas. However these engines have limited suitability for use in mobile high horsepower applications such as highway trucks and are generally significantly less efficient than compression ignition (diesel) engines. This significantly lower efficiency is primarily due to the lower compression ratio used in engines that compress a fuel mixture in the cylinder to prevent pre-ignition of the fuel mixture before the spark plug is fired. In heavy duty highway truck applications, a typical modern spark ignition engine will have a compression ratio around 11, whereas the compression ratio of a modern diesel engine will be between 16 and 18.
Over the past two decades a number of compression ignition engines have been developed to use diesel and natural gas together. These engines may utilise separate or integrated injectors to introduce streams of diesel and natural gas into the cylinders. The diesel is typically a “pilot” fuel that provides ignition of the natural gas—the gas being the majority of the fuel in the combustion process. Engines must be specially designed to accommodate two injectors or, alternatively, very complex single injectors utilising parallel or concentric injection paths may be used. Regardless of injector type, high pressure gas injectors generally suffer accelerated wear due to the absence of effective lubrication in the fuel. Also, diesel injectors may limit the minimum diesel flow due to turndown limitations in their design or have poor atomisation characteristics at low diesel flows.
Where direct (cylinder) gas injectors have been developed they have universally suffered from high wear rates due to the absence of lubricating qualities in natural gas fuel.
Further, a number of gas/diesel fuel systems have been developed wherein the primary fuel for the engine is diesel, and natural gas is used to supplement the diesel fuel. In these systems the natural gas is introduced with the combustion air and compressed in the cylinder. These systems have only limited ability to displace the primary diesel fuel, and when the proportion of diesel displacement is pushed above 50% they are prone to pre-ignition and detonation problems which can cause serious engine damage. Additionally at diesel displacement levels around 50%, the economics of converting engines to gas are poor.
There is therefore a need for an improved vehicle fuel system.